The Autobots would likely be very surprised at first. Bumblebee might be confused but also very protective. He'd probably try to keep Optimus safe from any potential threats. Ratchet, as the medic, would be both concerned and intrigued. He'd want to make sure Optimus is healthy during the pregnancy.
Optimus Prime himself would have a complex reaction. He'd be worried about how this would affect his leadership and his ability to fight for the Autobot cause. But at the same time, he'd feel a strong sense of responsibility towards the new life. His closest friends among the Autobots would gather around him, offering their strength and support, and together they'd face the challenges that this strange situation brings.
The Decepticons' reactions could vary. Megatron might see it as an opportunity to weaken Optimus Prime, either by trying to harm the unborn or using the situation to spread chaos among the Autobots. Some of the more sympathetic Decepticons might actually be in awe, as this is such an unexpected event. And for the human allies, they would be shocked but also offer whatever support they can, like providing a safe place on Earth if needed.
The other characters in the Highschool DxD world would have a wide range of reactions. The angels might see Optimus as an unknown factor and be a bit suspicious. The fallen angels might try to manipulate the situation to their advantage. And the normal human students would be terrified at first. But as the story unfolds and they see Optimus Prime's noble actions, they could start to look up to him as a hero, much like how the Autobots are seen as heroes in their own universe.
The ponies' reactions would vary greatly. Rarity might be fascinated by Optimus Prime's design and might even try to make some sort of fashionable accessory inspired by him. Applejack would be more practical and would wonder what he was doing in Equestria and if he had any good intentions. Optimus would see the ponies as a new and interesting species. He would be patient with their initial wariness of him and would try to show them that he means no harm. He might also be a bit taken aback by their colorful and somewhat chaotic world compared to the more structured world of the Autobots.
One idea could be to explore how they prepare for the arrival of their new 'bot. They might have to make adjustments to their living quarters in Cybertron. For example, they could be seen building a special nursery area with all the latest technology to ensure the well - being of their soon - to - be - born child.
Baby Optimus Prime's character can be developed by exploring his relationships. If he has a mentor figure among the Autobots, his interactions with that character can shape him. He might look up to this mentor and try to imitate their good qualities. His relationship with other baby Transformers or younglings can also be important. He could learn about sharing, cooperation, and friendship through these relationships. Additionally, his discovery of his own strength, both physical and mental, can be a key part of his development. For instance, when he first realizes he can lift a small object that others thought was too heavy for him, it gives him confidence.
Optimus Prime could fit in as a powerful ally. He would be like a super - powered being in the RWBY world. His presence could shake up the power balance. Maybe he arrives through a portal and immediately starts helping the RWBY teams fight the Grimm. He could be seen as a symbol of hope, much like the Huntsmen and Huntresses are in RWBY.
One possible plot could be that Optimus Prime discovers he's pregnant through some sort of strange energy transfer. He then has to deal with the reactions of his fellow Autobots. Some might be shocked, while others try to support him. Another idea is that the pregnancy is a result of an experiment by the Decepticons gone wrong, and Optimus has to protect himself and the unborn life from their attempts to use it for evil.
It's possible. Fanfiction often takes existing characters and gives them new storylines. In the case of Optimus Prime and Arcee, a 'pregnant' fanfiction could be about them having a new form of life or a project that they are nurturing together, which is presented as a pregnancy - like concept. You may need to do some digging through various fanfiction communities and websites to find one that suits your taste and is suitable for all viewers.
To write their relationship development, start with their initial reactions to the pregnancy. Maybe Optimus is initially overprotective and Elita One has to remind him she's still a strong warrior. Then, as the pregnancy progresses, they could have small, tender moments like sharing memories of their past battles and how they hope their child will be like them in some ways.
Writers often develop Optimus Prime as a wise and noble leader. They show his strategic thinking in battles and his moral compass when dealing with humans. For example, he might make decisions that protect both his Autobot comrades and the humans at the same time. Bumblebee, on the other hand, is usually portrayed as more youthful and energetic. His beeping and use of radio snippets to communicate add a unique charm. Writers might have him be more impulsive at times, especially when it comes to protecting his friends, whether they are Autobots or humans.
To develop their characters further, you could explore their backstories more deeply. For example, show what made Optimus Prime become the noble leader he is, and the same for Elita One. Maybe there were some tough experiences in their past that shaped their personalities.